The present invention relates to a shaft for golf clubs (hereinafter referred to simply as shaft). More specifically, the present invention relates to a shaft that is 35-50 percent lighter than conventional shafts while providing the same outer diameter and the same characteristics as conventional shafts such as flexural rigidity, flexural strength, torsional rigidity, torsional strength, and crushing strength.
In one type of golf club, a fiber-reinforced composite material (hereinafter referred to as FRP) is used in forming the shaft. In this type of shaft, a fiber-reinforced fiber material is formed by lining up reinforcing fibers in a “one-directional” pre-impregnation (hereinafter referred to as prepregs) and then immersing the aligned fiber material in a resin. The shaft is then formed by wrapping the fiber-reinforced material around a tapered metal mandrel and hardening the composite in a laminated state. This type of golfclub shaft is widely used due to its high specific rigidity, specific strength, and the degree of freedom allowed in its design.
FRP shafts often use a two-layer structure to form the reinforced composite. An inner layer is formed of angled fibers (angled layer) and an outer layer is formed from straight fibers (straight layer). In the angled layer, prepregs are glued together so that the reinforcing fibers form angles of +theta, −theta relative to the longitudinal axis of the shaft. In the straight layer, the prepregs are stacked so that the reinforcing fibers are within a +/−20 degree range relative to the longitudinal axis of the shaft.
In recent years, there has been a trend toward creating lighter golf club shafts. By lightening the shaft it is possible to produce a larger “sweet spot” in the golf club head. With a larger “sweet spot” in the golf club head, golf clubs can be designed to accompany higher head speeds, longer shafts, and larger heads.
Conventionally, lighter golf club shafts are designed and manufactured by simply reducing the number of straight layers and angled layers that make up the shaft. As a consequence of reducing the number of layers there is a reduction in flexural rigidity, flexural strength, torsional rigidity, torsional strength, and crushing strength. These reductions in strength and rigidity are undesirable.
Alternative methods have been attempted to create lighter shafts which minimize the adverse effects on strength and rigidity. Two methods which provide for a lighter shaft while maintaining flexural rigidity and torsional rigidity are as follows:                (1) reduce the number of straight layers and/or angled layers while also using a reinforcing fiber that has a high elasticity in these layers; and        
(2) reduce the thickness of the layers by changing the shape of the shaft itself, primarily by increasing the outer diameter.
In method (1), the flexural rigidity and torsional rigidity are comparable with conventional shafts. However, reinforcing fibers with high elasticity generally have low strength. Golf club shafts designed according to method (1) result in flexural and torsional strengths which are the same as, or even lower than, golf clubs shafts which simply have the number of layers reduced.
In method (2), increasing the outer diameter near the grip is effective in maintaining flexural rigidity. However, the increased grip diameter results in a golf club shaft that is difficult to handle, making the arrangement impractical.
Japanese laid-open utility model publication number 62-33872 discloses a method for improving the torsional rigidity and torsional strength in FRP shafts. According to this method, an FRP shaft includes angled layers and straight layers which are formed with the angled layer as the outermost layer. However, the finishing process of the FRP shaft, i.e., polishing and the like, can result in a loss in the angled layer. The thickness of the angled layer is needed to maintain torsional rigidity and torsional strength. Thus, FRP shafts made according to this method do not have consistent quality. In addition, this method does not provide for a lighter FRP shaft.
Japanese laid-open patent publication number 8-131588 provides for another method of improving an FRP shaft. According to this method, an FRP shaft includes (starting from the inner most layer): a thin hoop layer, a straight layer, and an angled layer. As in the method previously described above, the finishing process of the FRP shaft, i.e., polishing and the like, can result in the loss of the angled layer needed to maintain torsional rigidity and torsional strength. Thus, FRP shafts made according to this method do not have consistent quality and do not result in a lighter FRP shaft.